1. Field
The present invention generally relates to the field of lithography tools and, more particularly, to alignment of a collector device in a lithographic apparatus.
2. Background
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., one or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus can include so-called “steppers,” in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time and so-called “scanners,” in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation (EUV) having a wavelength, for example, within a range of about 13-14 nm. It has further been proposed that radiation with a wavelength of less than about 10 nm could be used (e.g., about 6.7 nm or 6.8 nm). In the context of lithography, wavelengths of less than about 10 nm are sometimes referred to as “beyond EUV” or as “soft x-rays.”
Extreme ultraviolet radiation and beyond EUV radiation can be produced using, for example, a plasma. The plasma can be created for example by directing a laser at particles of a suitable material (e.g., tin), or by directing a laser at a stream of a suitable gas or vapor (e.g., Xe gas or Li vapor). The resulting plasma emits EUV (or beyond EUV radiation), which is collected using a collector such as a focusing mirror or a grazing incidence collector.
The orientation and/or position of the collector will determine the direction in which radiation is directed from the collector (e.g., reflected from the collector). Radiation will need to be accurately directed to different parts of the lithographic apparatus, and it is therefore important for the collector to direct radiation in a specific direction. When a lithographic apparatus is constructed and used for the first time, it may be possible to ensure that the collector directs radiation in such specific direction. However, over time it can be difficult to ensure that the radiation beam is always directed in this specific direction. For instance, movement of parts of the lithographic apparatus (e.g., parts of the radiation source) can shift the direction of radiation. Additionally or alternatively, when parts of the lithographic apparatus are replaced (e.g., for maintenance purposes) even a slight misalignment of replacement parts can shift the direction of radiation. It is therefore desirable to align or re-align a collector of a radiation source and parts of the lithographic apparatus located further along the path of the radiation beam. Since an illuminator (hereinafter also referred to as an “illumination system” or “illumination arrangement”) is a part of the lithographic apparatus that receives radiation directed by the collector, it is desirable to align or re-align the collector of the radiation source with respect to the illuminator.
A proposed method of aligning the collector with respect to the illuminator involves attaching light emitting diodes (LEDs) to the collector. A measurement of radiation emitted by the LEDs can be used to determine an orientation (e.g., tilt) and/or position of the collector with respect to a default (or reference) position. However, an issue with this method is that the LEDs may not be robust to withstand a harsh environment surrounding the collector. For instance, high temperatures and prolonged exposure to EUV radiation can quickly damage or destroy the LEDs. Furthermore, the LEDs must be attached to the collector with a high degree of accuracy, with little or no drift in the position of the LEDs over time. Given these conditions, an LED-based implementation is difficult to achieve
A lithographic apparatus and method for alignment of a collector device with respect to an illuminator device is needed to solve the aforementioned issues.